SAR HS 300 Lecture Notes - Lecture 16: Chickenpox, Protozoa, Microorganism
6 Aug 2019
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HS300 Lecture 16
I. Diagram of Epidemiologic Triangle
II. Measures of Disease Outbreaks
A. Attack Rate
1. A cumulative incidence rate used when the nature of the disease or
condition is such that a population is observed for a short period of
time.
2. Population-at-risk increases greatly over a short period of time.
3. Relatively short period of time.
4. Uses for Attack Rates:
a) In practice, attack rates are commonly used to describe
occurrence of:
(1) Food borne illness.
(2) Infectious diseases.
(3) Acute epidemics.
5. Example: a flu outbreak occurred in a residential high school, that
housed 200 students. Case A diagnosed Oct 1st, and case B on Oct 2nd.
10 days later (Oct 11th), 25 additional cases occurred.
Epidemiologists believed that the second group of cases represented a
new generation of cases, following exposure to the primary cases, case
(A and B). No students were immune to the flu prior to this outbreak.
Calculate the overall attack rate and secondary attack rate for this flu
outbreak.
Attack Rate =
Secondary Attack Rate =
B. Secondary Attack rate
1. An index of the spread of disease within a family, household, dwelling
unit, dormitory or similar circumscribed group.
2. A measure of contagiousness.
3. Used to evaluate measure in infectious disease control.
4. Index case = case that first comes to the attention of public health
authorities.
5. Initial case(s) = Index case(s) + coprimaries.
6. Coprimaries = cases related to index cases so closely in time that they
are considered to belong to the same generation of cases.
7. “Sentinel” case or event = environmental health.
C. Case Fatality Rate
1. The number of deaths caused by the disease among those who have
the disease.
2. Examples of diseases with a high CFR are rabies, cholera, and
meningitis.
3. Sample Calculation
a) Assume that an outbreak of meningitis occurs in New South
Wales. Health authorities record 98 cases of the disease, all of
whom are untreated. Among these, 60 deaths are reported.
CFR =
b) In a meningitis outbreak in New South Wales, the reported
case fatality rate was 61.2%.
III. Terms:
A. Endemic: usual/typical level of disease occurrence in a population.
B. Epidemic: unusually high level of disease occurrence in a population in the
light of past experience.
C. Pandemic: a worldwide epidemic.
IV. Agents
A. Microbial Agents of Infectious Disease
1. Bacteria (strep).
2. Viruses (influenza).
3. Mycoses - fungal diseases (athlete’s foot).
4. Protozoa (malaria).
5. Helminths (roundworm).
B. Characteristics of Infectious Disease Agents
1. Infectivity
a) The capacity of an agent to enter and multiply in susceptive
host.
b) Measured by the attack rates, specifically secondary attack
rates.
c) High infectivity = when secondary attack rate of a disease is
high = measure of contagiousness.
2. Pathogenicity
a) Capacity of the agent to cause disease in the infected host.
